Optocouplers are commonly used to provide electrical isolation between high and low voltage portions of a circuit. Optocouplers usually include a light-emitting diode (LED) and a light-responsive transistor. Electrical isolation occurs because information is transmitted using light emitted by the LED and received by the light-responsive transistor. Optocouplers are also commonly used in circuits as indicators, or as control devices, wherein the light-responsive transistors are energized to control other circuits.
Circuits for monitoring line-voltage switching devices typically monitor only one AC line-voltage switching device (e.g., a switch, thermostat, contactor, etc.) which switches an AC line voltage to an electrical load (e.g., a heater, air conditioner, ice-cube maker, etc.). An input circuit including an optocoupler reads the state of one switching device and generates a single output signal representative thereof. The output signal is coupled to an input pin of a processing circuit (e.g., a microprocessor, microcontroller, programmable logic array, etc.). The processing circuit reads the input pin and determines whether the load is on or off based on whether the input pin is logic low or logic high.
In order to monitor multiple AC line-voltage switching devices, however, the input circuit is usually duplicated for every AC line-voltage switching device being monitored. This duplication of the optocoupler and other circuit components increases cost and requires the use of multiple input pins on the processing circuit. It would be advantageous to design a monitoring circuit capable of reading multiple AC line-voltage switching devices without the necessity of duplicating the input circuit for each switching device and without the need for a single input pin on the processing circuit for each switching device.
In many types of equipment, a synchronization circuit is typically used to generate a sync signal from the AC line voltage to provide a highly-accurate timing signal. The sync signal is generally a 50 Hz or 60 Hz square wave signal with logic low and logic high states which is processed by the processing circuit. This type of circuit typically keeps time in a processor-controlled system where the clock-source (e.g., a sloppy RC network) for the processing circuit is not stable. It would also be advantageous to combine the information provided by the synchronization circuit (i.e., whether the AC voltage source is in a positive AC half-cycle or a negative AC half-cycle) with a single input circuit (instead of several input circuits) to monitor multiple AC line-voltage switching devices.